Common Issues with Circulation Pumps in ATTACK Boilers and Their Causes

What to do: If the pump is new or relatively young, try manual unblocking. On the PR 25/6 pump, there is a screw on the front side (usually a hex key 6 mm or flat screwdriver), which covers the rotor housing. After unscrewing and draining a small amount of water, the rotor shaft is visible – it can be carefully turned with a flat screwdriver or a rubber mallet can be tapped through it. If the rotor starts moving, the system should be flushed and a magnetic filter installed. If the rotor is damaged (corrosion, mechanical damage), the pump must be replaced – for example, the ATTACK Spare Part – Pump RS 25/6-3-PR, 180 mm is suitable for boilers with a 180 mm shaft distance.

2. Air in the Circulation Pump and Heating System

The second most common failure – and at the same time the one with the most varied manifestations. Air in the system can cause everything from pump noise (grinding, gurgling, cracking) to complete circulation stoppage, because an air pocket in the pump body blocks the hydrodynamic flow.

Air enters the system through several paths:

• During system filling – if the system was drained (for repair, pump replacement, or winterization) and refilled, air pockets were not properly bled out.
• Insufficient pressure in the expansion tank – if the system pressure drops below 0.5 bar, air from the system desorbs from the water. ATTACK boilers have a pressure gauge – monitor it regularly.
• Damaged expansion tank – a cracked membrane causes pressure fluctuations and air bubbles.
• Air intake through leaks – on the suction side of the pump (before the pump), there may be a slight vacuum, and if there is a leaky connection, air is sucked into the system.
• Corrosion processes – chemical reactions in the system can release gases (especially hydrogen during corrosion of zinc from galvanized pipes).

How to bleed the pump: most circulation pumps for ATTACK boilers have a bleed screw on the front (a small screw in the center of the front cover). Before bleeding, place a cloth or container under the pump, turn the screw half a turn, and wait until air stops coming out and only water flows. Then tighten the screw. In modern electronic pumps such as the WILO Yonos PARA (e.g., ATTACK Pump WILO Yonos PARA RS25/6, 130 mm), an automatic air venting function is sometimes available, but it is not reliable with large air pockets – manual bleeding is always more reliable.

3. Electrical faults of the pump

Electrical faults rank third. These include a variety of problems – from burnt windings through capacitor failure (in older models) to issues with control electronics (in modern EEI class A pumps).

3a. Burnt motor winding

A classic fault in older pumps with single-phase asynchronous motors. It manifests as complete non-functionality – the pump is completely silent, no buzzing sound, but power is present at the terminals. Measuring the resistance of the winding with a multimeter will show an open circuit or short. Causes:

• Long-term operation with a blocked rotor (winding overheated)
• Voltage fluctuations in the grid (overvoltage, undervoltage)
• Moisture in the electrical part (condensation in the motor compartment)
• Simply aging – the average lifespan of a winding is 15–20 years under normal operation

3b. Fault in the control electronics of class A pumps

Modern class A efficiency pumps (such as ATTACK Heating Circulation Pump PR 25/6, 180 mm, Class A or ATTACK Heating Circulation Pump 25/6, 130 mm, Class A) use frequency converters and PWM control. This electronics is more sensitive to voltage spikes and electromagnetic interference, as well as condensation. Symptoms:

• The pump displays an error code on the display or LED
• The pump restarts, then fails again after a while
• The pump operates only at lower speeds, and shuts down under higher loads
• Incorrect power or flow measurement (in pumps with integrated measurement)

To learn what class A actually means and what benefits and risks it brings, read the article Energy Efficiency Class A in Circulation Pumps: What It Means in Practice.

3c. Poor contact or damaged cable

A trivial, yet surprisingly common cause: a loose or corroded contact in the pump terminal box. In the boiler room environment, thermal fatigue of connections, condensation, and oxidation occur. Measuring voltage directly at the pump terminals (not just at the circuit breaker) will show whether the issue is in the power supply. If the tester lights up and there is no voltage – the problem is in the cable or fuse, not in the pump itself.

4. Pump noise – cavitation, bearing wear, and mechanical faults

A noisy pump does not necessarily mean it is non-functional, but it warns of an impending fault. We distinguish several types of noise, each with a different cause.

Cavitation in the pump

Cavitation occurs when the pressure on the suction side of the pump drops so low that vapor bubbles form in the pumped water (water locally "boils" at normal temperature). These bubbles instantly implode on the discharge side, causing characteristic thudding and vibrations. Long-term cavitation mechanically damages the impeller and rotor surface – essentially like sandblasting at a micro level. Causes of cavitation in pumps in ATTACK boilers:

• Too low system pressure (below 0.8–1.0 bar)
• Too high water temperature at pump inlet (pump is installed on the hot side without a protective loop)
• Clogged filter/strainer before the pump – causes excessive pressure drop on the suction side
• Pump running at too high speed with insufficient flow (e.g., closed zone valves)

Wear of bearings and shaft seals

Pumps with wet rotors (i.e., all common types for ATTACK boilers) do not have classical dry ball bearings – instead, they have bearings made of graphite or ceramic, which are lubricated and cooled by the heating water itself. These bearings are very durable with clean water, but they wear out quickly with contaminated water (abrasive particles). The first symptom is a sandy sound, followed by vibrations, and finally mechanical destruction of the shaft. Ceramic bearings are usually replaceable, but for most modern compact pumps, it is economically more advantageous to replace the entire pump.

5. Water leaks – sealing faults of the pump

Water leakage at the pump can have two sources: a damaged seal of the pump itself (between the motor cover and the hydraulic part) or leaking connections/fittings through which the pump is connected to the piping. It is important to distinguish these two cases, as the repairs differ.

Leaks from the pump seal (so-called mechanical seal or gland) are less common in wet rotor pumps, as these pumps have only a static seal (O-rings) between the hydraulic body and the motor body – they do not have a moving shaft seal (the rotor is directly in contact with the water). However, O-rings age, harden, and crack, especially with repeated thermal stress. Common signs: slight dampness around the pump flange at operating temperature, which disappears after cooling down.

Leaks through fittings usually appear shortly after pump replacement – either the seal was not properly seated, an unsuitable gasket was used, or the fittings were not tightened properly. More about this issue can be found in the article Installation of a circulation pump in an ATTACK boiler: procedure and common mistakes.

6. Insufficient pump performance – circulation is happening, but insufficiently

A less dramatic, but equally harmful fault: the pump is operating, but the water flow is too low. It manifests as uneven heating (some radiators are cold), a long warm-up time of the system, or repeated boiler overheating despite apparently functional pump.

Specific causes of insufficient performance in practice:

• Clogged strainer before the pump – this is an extremely underestimated cause. The strainer (Y-filter) is located before the pump at the inlet, and if not cleaned regularly, it eventually clogs and blocks the flow. Customers often don’t even know they have one, let alone clean it. Solution: regular inspection and cleaning (at least once a year).
• Incorrect speed setting – mechanical three-speed pumps have a switch 1/2/3. If speed 1 is set for a large system, the flow will be insufficient. Electronic pumps may have a setting in "quiet" mode, which limits performance.
• Clogged piping – in older homes with galvanized piping, the pipes can be so clogged with corrosion products that even a properly selected pump cannot overcome the blockage. Only chemical flushing or pipe replacement helps here.
• Too high hydraulic resistance – incorrectly dimensioned system (pipes too narrow for the heating area). A pump 25/6 has a limited head of 6 m and a flow of 2.5–2.8 m³/h – if the system requires more, the pump physically cannot keep up.

7. Fault caused by unsuitable quality of heating water

This is a systemic cause of many pump faults, but it is often overlooked because it is not immediately visible. Heating water should have the following properties:

• Hardness below 2.0 °dH (ideally demineralized water with corrosion inhibitor added)
• pH in the range of 7.5–9.0 (alkaline water slows down corrosion of steel components)
• Chloride content below 50 mg/l (chlorides cause pitting corrosion of stainless steel)
• Zero oxygen content (closed system, properly adjusted expansion tank)

If the water is unsuitable (too hard, acidic, rich in chlorides, or has a high oxygen content), the result is accelerated corrosion inside the pump, lime deposits in the pump body and on the rotor, magnetic corrosion (magnetite), and biological deposits (bacteria in low-temperature systems). This leads to early rotor seizure, noise, reduced performance, and eventually physical damage.

In practice, we see with customers who have very hard water and top up the system directly from a tap (which is not allowed), significantly shorter pump lifespan – instead of the usual 10–15 years, the pump fails within 3–5 years. A magnetic filter (e.g., type Spirovent or Fernox) installed before the pump can capture magnetite and significantly extend the pump's lifespan.

8. Failure after pump replacement – what can go wrong during installation

An interesting category includes failures that appear shortly after pump replacement. In such cases, either the new pump was not selected correctly (wrong dimensions, wrong performance), or an error occurred during installation. The article Pump size 130 mm vs 180 mm: which fits my boiler describes in detail why confusing these dimensions is critical. Briefly: different generations and types of ATTACK boilers may have prepared holes either for a 130 mm or 180 mm shaft distance. If you install a pump of the wrong size, it will not fit physically or will not be properly sealed, resulting in leaks.

Another typical installation error: the pump is installed in the wrong position. Most circulation pumps can be installed horizontally (shaft horizontal) or vertically (shaft up or down). However, when installed with the shaft pointing down, the bearings wear out faster and in some models, oil leakage may occur. Always check the manual for the specific model.

For those dealing with the replacement of the WILO Yonos PARA pump in an ATTACK boiler, the article Replacement of the WILO Yonos PARA pump in the ATTACK boiler: what you need to know is relevant. And if you are looking for specific spare parts, see for example ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm for boilers with a 180 mm shaft distance.

9. Seasonal and operational failure patterns

From practice, we know that pump failures in ATTACK boilers do not occur evenly throughout the year. There are typical seasonal patterns:

• September/October – Highest number of service calls. After summer, the boiler is put under operating pressure and temperature for the first time. A seized rotor will show up immediately. We always recommend checking the pump manually by turning it before the season.
• December/January – Failures due to long-term load. Pumps operate at full capacity 24/7. Bearing wear, overloading of the electrical system.
• Spring (March/April) – Detection of dirt during the final cleaning of the season. A good time for inspection and filter replacement.

Practical tip: every year at the beginning of the season (before the first heating), perform a simple check: check the system pressure (should be 1.0–1.5 bar when cold), bleed the pump, manually turn the rotor (via the screw on the front), and check if it is not seized. This 15-minute task can save you from a service technician visit in December.

10. Pump failure vs. failure of another device – differentiation

Experienced technicians know that the symptoms of a "non-functional pump" may actually stem from a completely different source. Here is a quick diagnostic procedure:

• Boiler overheats, but the pump is working → look for a problem in the thermostatic valves, zone valves, or a clogged boiler heat exchanger, not the pump.
• Pump is working, but some radiators are cold → hydraulic imbalance, clogged thermostatic valves on radiators, or air in the radiators. The pump is not the culprit.
• Pump is silent, boiler overheats → the pump is not working (electrical or mechanical failure). Check the power supply, then the rotation.
• Boiler shuts down, pump is working, room temperature is normal → boiler regulation or thermostat sensor failure. The pump is not the problem.

Many service calls would be unnecessary if customers could distinguish between these scenarios. That is why the educational section – including this article – is so important.

Pump lifespan in ATTACK boilers

The average lifespan of a circulation pump in an ATTACK boiler, with proper operation, clean heating water, and a magnetic filter, is 12–18 years. Without a magnetic filter or with hard water, it may be 5–8 years. Modern electronic pumps of class A have more complex electronics that may fail sooner (7–12 years), but their lower energy consumption and quieter operation compensate for this. More on the topics of lifespan and signs of an impending failure can be found in the article When to replace the circulation pump in your boiler: signs and lifespan.

When deciding whether to repair or replace a pump, a simple rule applies: if the pump is older than 10 years and the repair would cost more than 40% of the price of a new one, it is worth buying a new one. A complete overview of available replacement pumps and original parts for ATTACK boilers can be found in the main ATTACK pump category on atria.sk.

Most frequently asked questions (FAQ)

The pump in my ATTACK boiler buzzes, but water is not flowing – what does it mean?

The most common cause is a seized rotor. The motor is receiving power (hence you hear the transformer buzz), but the rotor is mechanically blocked by deposits or has been idle for a long time. Try unscrewing the cover on the front of the pump and gently turn the rotor shaft with a flathead screwdriver. If it turns, bleed the system and check the quality of the heating water. If the rotor is firmly seized or damaged, the pump needs to be replaced.

Why does my new pump make a grinding noise right after installation?

Typically, it is air that remains in the system after draining and refilling. The new pump circulates it when it starts. Solution: fully bleed the pump through the bleed screw on the front, then bleed all radiators in the house and check the system pressure (should be 1.0–1.5 bar when cold). If the grinding persists for more than 24 hours, check if air is entering somewhere due to a leak.

What is the cause if the pump works well in the morning, but stops in the evening?

This symptom pattern typically indicates thermal failure of the electrical part – the winding or electronics of the pump overheats during operation and the protection disconnects the pump. In the morning, when the pump is cooled down, it starts normally again. The cause may be a weak winding nearing the end of its life, a cooling issue with the electronics, or an excessively high ambient temperature in the boiler room. The pump needs to be replaced – predictable failure at the worst time (during freezing weather) is a worse alternative.

Can I install a 180 mm pump into an ATTACK boiler originally equipped with a 130 mm pump?

No, the dimensions are not interchangeable without further modifications. The shaft distance (distance between the axes of the threaded connections) must exactly match the distance of the connections on the boiler. If you were to install a pump with the wrong shaft distance, you would either not be able to connect it physically or would have to use extension hoses/adapters, which is not a suitable solution. This topic is addressed in detail in the article Pump size 130 mm vs 180 mm: which fits my boiler.

The pump in my ATTACK boiler is dripping at the flange – should I replace it immediately?

Not always. First, check whether the dripping comes from the pump's own sealing (O-ring between the hydraulic and motor body), or from the pipe connections. A loose connection can be easily fixed by tightening it in a warm state (when the system is under pressure). If the problem is the O-ring, in some models it is possible to replace only the seal without replacing the entire pump. However, if the pump is old and the O-ring has cracked due to age and thermal fatigue, it is often more practical to replace the entire pump. For leaks in ATTACK pumps with a 130 mm spacing, see ATTACK Heating circulation pump 25/6, 130 mm, class A as a variant with new seals in one package.

Is it worth replacing only the pump rotor, or should the whole pump be replaced?

It depends on the specific model. In some older three-speed pumps, the rotor (so-called wet rotor with a tube) was available as a spare part at a reasonable price. In modern electronic pumps (WILO Yonos PARA and similar), the rotor is an integral part of the hydraulic section and is delivered only together with the motor block, which is effectively the whole pump. In practice: if a service kit with the rotor is available for less than 50% of the price of a new pump and the pump is less than 8 years old, the repair is worth it. Otherwise, a full replacement is better.

Conclusion: prevention is always cheaper than repair

Failures of circulation pumps in ATTACK boilers most often have identifiable causes – dirty water, long periods of inactivity, air in the system, unsuitable selection or installation. Most of them can be prevented by regular inspections (at least once a year), maintaining the correct system pressure, keeping the heating water clean, and installing a magnetic filter. If a failure does occur, correct diagnosis – based on the symptoms described in this article – will save you time and unnecessary expenses on incorrect spare parts. And if the pump is beyond repair, in the category of replacement pumps for ATTACK on atria.sk you will find the correct type for your boiler, including original WILO Yonos PARA pumps and type-appropriate alternatives.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.